metallurgy

Metallurgy is the science and technology of metals, concerned
with their extraction from ores, the methods
of refining, purifying, and preparing them for use and the study of the
structure and physical properties of metals and alloys.
A few unreactive metals such as silver and gold are found native (uncombined), but
most metals occur naturally as minerals (i.e., in chemical combination with nonmetallic elements). Ores are mixtures
of minerals from which metal extraction is commercially viable.

Over 5,000 years man has developed techniques for working ores and forming
alloys, but only in the last two centuries have these methods been based
on scientific theory. The production of metals from the ores is known as
process or extraction metallurgy; fabrication metallurgy concerns the conversion
of raw metals into alloys, sheets, wires, etc, while physical metallurgy
covers the structure and properties of metals and alloys, including their
mechanical working, heat treatment, and testing. Process metallurgy begins
with ore dressing, using physical methods such as crushing, grinding, and
gravity separation to split up the different minerals in an ore. The next
stage involves chemical action to separate the metallic component of the
mineral from the unwanted nonmetallic part. The actual method used depends
on the chemical nature of the mineral compound (e.g., if it is an oxide
or sulfide, its solubility in acids, etc.) and its physical properties.
Hydrometallurgy uses chemical reactions in aqueous solutions to extract
metal from ore. Electrometallurgy uses electricity for firing a furnace
or electrolytically reducing a metallic compound to a metal. Pyrometallurgy
covers roasting, smelting, and other high-temperature chemical reactions.
It has the advantage of involving fast reactions and giving a molten or
gaseous product which can easily be separated out. The extracted metal may
need further refining or purifying; electrometallurgy and pyrometallurgy
are again used at this stage. Molten metal may then simply be cast by pouring
into a mold, giving, e.g., pig iron, or
it may be formed into ingots which are then hot or cold worked, as with,
e.g., wrought iron.

Mechanical working, in the form of rolling, pressing, or forging,
improves the final structure and properties of most metals, it tends to
break down and redistribute the impurities formed when a large mass of molten
metal solidifies. Simple heat treatment such as annealing also tends to remove some of the inherent brittleness of cast metals.